When a power transistor is in the current-conducting lo state, drive circuits are needed to provide a level of base current Which ensures that current conduction is present in the transistor no matter its operating condition. Also, there is a need to keep the forward power losses to a minimum. This is generally achieved by providing a substantial base current that puts the transistor in a state of full or over saturation.
However, when operating a power transistor in this manner, the amount of expended drive power is quite substantial. When the edge of saturation of the transistor is reached, additional increases in the base current do not result in an improvement of the conducting state nor in a reduction of the transistor on-state voltage. Rather, these increases in the base current only cause the drive-power to be increased. In addition, non-conducting power transistors are powered up unnecessarily when operated in this manner in pwm-controlled converter circuits. If counter-electro magnetic forces result, as in the case of an inductive load driven by a pwm-controlled ac converter, at least one of the power transistors could pass over into the undesirable, inverse direction of operation. This transistor could be destroyed during current commutation operations due to sudden, steep changes in the output voltage. ("dV/dt-loading")
Operating a power transistor in the over or full saturation condition leads to considerable problems when the transistor is switched off. In order for the load current to be able to dissipate through the transistor after it has begun to switch off, all charge carriers must first be withdrawn from the base region and the base-emitter junction must be blocked. The amount of time this takes is a dead time called storage time. The deeper into saturation the transistor was before the transistor is forced to switch off, the longer is the storage time. The duration of this storage time influences the losses which occur during the switching-off operation. For this reason, self-regulating drive circuits are designed so that when the power transistor is switched on, the operating point is as close to the edge of saturation as possible. In other words, the operating point is in the area of quasi-saturation.
In this connection, a self-regulating driver stage is shown in the publication ELEKTRIE, Berlin 39 (1985) 5, page 180, illustration 7. Here, an anti-saturation diode is used to avoid full or super saturation. In this method, the voltage on the output of the transistor is limited to the sum of the voltage on the base-emitter junction and the conducting-state voltage of the anti-saturation diode. In the final control element of the transistor, excess base currents which are not needed to reach the edge of saturation can be dissipated into the collector-emitter junction of the transistor. A disadvantage with this type of circuit is that excessive control losses occur constantly. In addition, the anti-saturation diode must be designed to conduct any possible high currents.
Another self-regulating driver stage is shown in FIG. 3 of the German Published Patent Application No. 32 15 009. Here, a series connection comprising a high-speed circuit breaker and a current regulator is arranged between an auxiliary power source and the base of the driven power transistor. The current regulator contains a power transistor whose base is coupled through an anti-saturation diode to the output of the s driven power transistor. There are disadvantages when using these self-regulating driver stages, because they depend directly on the load current in the driven power transistor. The base-current closed loop is unstable due to the unavoidable line inductances, the base-emitter capacitance, and the conducted interferences in the drive power transistor. A powerful harmonic oscillation occurs constantly in the base current. These types of oscillations can be reduced by placing a damping resistor at the base lead of the power transistor, but the damping resistor diminishes the effectiveness of the anti-saturation diode.
Another drive circuit is described in U.S. Pat. No. 4,755,741. This drive circuit is connected to the driven transistor at its base lead through a second emitter lead. The transistor is driven by a final control element that is connected in series with a constant current source and another, variable current source that is controlled by the magnitude of the load current. This variable current source operates independently of the actual magnitude of the load current, and results in an operating point in the saturation or over saturation range. An operating point at the edge of saturation is possible because the excess portion of the drive power is dissipated from the base of the driven transistor through a second emitter electrode. A disadvantage of this type of arrangement is that when the power balance of the circuit worsens, an excess current is continuously fed into the base of the driven transistor. This excess current is fed back into the drive circuit through a short-circuit with a second emitter electrode being used for overflow storage. This method of removing excess current is also known as a desaturation bypass. It is not possible to adjust the operating point, especially at or around the edge of saturation, since the dissipation of excess base current depends on the external and internal design of the driven transistor.
Thus there is a need for a device to control the operating point in a power transistor that is retrofittable to already existing drive circuits and avoids the above-mentioned disadvantages associated with the known circuits.